Most pressure vessels are equipped with a relief valve to prevent catastrophic rupture in case of excessive temperature or pressure. Should the pressure vessel be in a fire, it is possible that the volume of liquid or gas contained therein will increase rapidly and reach a situation where the pressure is higher than it was at room temperature. For cylinders constructed of aluminum, Kevlar or plastic, this can pose a very critical problem. As the temperature increases, the material of which the container is constructed tends to stretch, soften or weaken and thereby is unable to hold the pressure for which the vessel was designed.
A number of temperature and pressure activated relief valves are taught in U.S. Pat. Nos. 2,040,776; 3,472,427; 4,059,125; and 4,352,365. In addition, pressure and/or temperature activated relief valves are currently commercially available through Sherwood-West Corporation of Irvine, Calif.; Rego Company of Chicago, Ill.: Baker Cat, Inc. of Belle Chasse, La.: and Superior Valve Company of Washington, Pa. Some of these valves use a fusible metal positioned within a narrow passageway which is designed to melt at a predetermined temperature and be blown out by the pressure of the substance contained within the pressurized cylinder. One of the drawbacks with using a fusible metal is that when the valve is assembled a number of voids can occur within the passageway filled with the fusible metal. These voids shorten the life of the valve by allowing the fusible metal to extrude into the voids, thereby creating possible leaks. If the device is a combination rupture disc/fusible metal device, the voids may minimize support of the disc and lead to premature failure. The presence of any foreign matter trapped within the fusible metal can also create a problem in that the relief valve is designed to release at a certain temperature or pressure. If foreign matter is present, the valve could release prematurely thereby minimizing its usefulness.
Now a method has been invented for forming a thermally activated pressure relief valve which assures that as the fusible metal is inserted into a helical passageway the presence of voids and foreign matter will be minimized of eliminated.